The Trump administration plans to temporarily reassign 120 F.B.I. agents in Washington to nighttime patrol duties as part of President Trump’s crackdown on the city’s street crime, according to people familiar with the matter.

Most of the agents will be pulled from their regular duties at the F.B.I.’s Washington field office, but it was not immediately clear if agency leaders would need to pull additional personnel from nearby cities, the people said, speaking on condition of anonymity to describe details of an effort that were not meant to be public.

In a statement, an F.B.I. spokeswoman said agents were “participating in the increased federal law enforcement presence in Washington,” and referred additional questions to the White House.

The temporary reassignments were first reported by The Washington Post.

Mr. Trump has said that crime in Washington is spiraling out of control. While statistics show that violent crime in the city hit a 30-year low last year and is down another 26 percent so far this year, youth crime has been a persistent problem for city officials.

Mr. Trump stepped up threats to put Washington under federal control last week after a prominent member of the Department of Government Efficiency was assaulted during an attempted carjacking. He has planned an announcement for Monday morning to unveil his plan to fight crime in the city. The administration had previously said that additional federal agents would be used to combat crime in Washington.

This is not the first time the Trump administration has decided to reallocate F.B.I. personnel to follow the president’s priorities. In recent months, roughly 900 F.B.I. agents around the country have been given temporary assignments to help with immigration enforcement.

It can be challenging for F.B.I. agents to make meaningful contributions to such assignments, given that the bureau’s agents are investigators not trained in patrol-focused policing. Many others are inexperienced in handling immigration cases. F.B.I. agents on such assignments are often used to provide additional security on the street, while other law enforcement officers do their jobs.